1. Characterizing Bias Current Spikes
Device Under Test
Input Current
Wideband TIA
A wideband transimpedance amplifier was used to directly view the input current of several chopper amplifiers

2. Characterizing Bias Current Spikes
OPA657
OPA2188
Bias Current (IB)
2 pA
160 pA
Current Noise (IN)
1.3 fA/rtHz
7 fA/rtHz
Bias current spikes should be apparent above other noise sources

3. Input Bias Spike Measurement Equipment
A shielded enclosure was used to mitigate extrinsic noise
Direct connection to the oscilloscope via coax cable
Spikes were viewed on a 500 MHz oscilloscope
50 Ohm input impedance
DC coupling (maximize bandwidth)
Averaging used to remove random noise

4. OPA2188 Spikes repeat at 2x the chopper clock frequency
Larger spike is due to the input commutation
Smaller spike is from the synchronous notch filter on the output
Largest spike peaks at 850nA
Total duration is ~24nS

5. OPA2333 Commutation frequency is much lower Different input topology
Input clock spikes are now of similar magnitude to notch filter spikes
Different input topology
Transmission gates reduce input current spikes
70 nA peak
216 nS duration

6. This is not unique to TI auto-zero topologies!
AD8639
1.7 uA!
This is not unique to TI auto-zero topologies!

7. Noise Feed-Through
Without proper design considerations noise from the input current spikes can appear in the output

8. Equivalent Schematic
Input current spikes can be viewed as current sources on the inputs
Input current spikes are outside the opamp bandwidth
The opamp can be removed to simplify analysis
Current spikes on the non-inverting input are not amplified

9. Contribution to Output Noise
Current spikes on the inverting input are coupled to the load by the feedback network
Output noise is dependant upon:
Input current spike magnitude
Feedback network impedance (RF and RG)
Load Impedance (RLOAD)

10. Noise Measurement
Output noise was amplified and viewed on a spectrum analyzer
OPA2211 with a gain of 11
HP3588 (10Hz to 150MHz)
Data collected using LabView
Shielded enclosure and cables
VRMS: Output voltage
AV: Gain of secondary amplifier
KN: Brickwall correction factor (1.056)
RBW: Resolution Bandwidth

11. Gain Effects on Total Noise
At high gains chopper noise is not a dominant noise contributor

12. Load Impedance Effects
High load impedances can exacerbate chopper noise from input current spikes

13. Feedback Network Impedance
Large feedback resistor values will also worsen the output noise

14. Output Filtering
Adding an RC output filter can mitigate noise seen by high impedance loads
COUT chosen to have an impedance much less than RLOAD at 2x chopping frequency
ROUT chosen to maintain opamp stability with the chosen COUT

15. Output Filtering
The corner frequency for the input current spike is actually much lower
The filter now includes the feedback resistance RF
Filter corner frequency can be chosen to remove noise without affecting desired signal

16. Output Filtering

17. Output Filtering
Harmonics due to input current spikes are completely eliminated
Autozero noise at chopping frequency is within the opamp bandwidth

18. Output Filtering OPA2188 Without Filtering OPA2188 With Filtering
Gain: 101, RF:10k, RG:100 Ohm
Oscilloscope 1MOhm input impedance is the load
Input current spikes are visible above other noise sources
OPA2188 With Filtering
Gain: 101 RF: 10k, RG: 100 Ohm
Oscilloscope 1MOhm input impedance is the load
Triggering oscilloscope becomes difficult due to low noise

19. Comparison to Non-Autozero Amplifiers
The noise level of a filtered chopper amplifier is on-par with non-chopper topologies

20. Minimizing Chopper Noise Effects
Input current spikes are not amplified by the part
Spikes on the inverting input will be coupled to the load by the feedback network
Minimize feedback resistance values
Reduces the voltage produced by current spikes
Standard design practice for low-noise, low-drift circuits
Load impedance directly contributes to the magnitude of voltage produced by the spike
An RC filter is an extremely effective way to reduce output noise
Corner frequency can be placed outside of the signal bandwidth
Noise through the feedback network experiences a much greater attenuation